1. Field of the Invention
The invention relates to a fluid pressure actuator of the type employed to effect movement of the operating stem of a valve relative to the valve housing, wherein the fluid pressure induced movement of the actuator is opposed by a spring, such as a stack of Belleville spring washers.
2. Description of the Prior Art
There are numerous fluid pressure actuators for valves that have been heretofore proposed. In substantially all cases, the fluid pressure actuator comprises a piston mounted on a shaft, which in turn is connected with the valve stem. Such piston cooperates with the interior walls of cylinder and the fluid pressure produced movements of the piston relative to the cylinder are opposed by a spring. In oil field operations, for example, it is desirable to employ Belleville springs to produce a very rapid increase in opposing spring force as a function of displacement of the piston. However, as is well known, Belleville type springs cannot be positioned where the edge of the spring can contact either the bearing or sealing surfaces of the cylinder or the piston shaft, because of the tendency of the spring edges to gouge or mar any surfaces which are disposed in closely surrounding relationship. It has therefore been necessary to provide special spring enclosures for fluid pressure actuators employing Belleville springs.
The total height or axial length of an actuator is also of concern. If the piston is to be moved through a distance by the fluid pressure, the stroke of the piston will be S and the axial length dimension of the actuator will generally be somewhat in excess of 2S, because of the length necessary to accomodate the solid height of the return spring under the piston in its lowermost position. If the further requirement that an indicating stem be provided on one end of the piston projecting out of the cylinder to provide an indication of whether the valve is in its pressure actuated position or in its spring biased position, the overall axial length of the actuator is increased to approximately 3S. It is therefore desirable to provide an actuator, including an indicating stem, wherein the total axial length of the actuator is not substantially in excess of two times the stroke of the piston.